It has long been accepted that minimizing deflector wobble is a necessary requirement for hologon laser scanners. Both dynamic and fixed mounts have been proposed. A fixed mount is shown in FIG. 1 and is also shown in U.S. Pat. No. 4,610,500 issued Sep. 9, 1986 to Charles J. Kramer. A dynamic mount is shown in FIG. 2. Dynamic mounts are described in U.S. Pat. No. 4,067,639 issued Jan. 10, 1978 to Charles J. Kramer and 4,353,615 issued Oct. 12, 1982 to Charles J. Kramer, et al. The effect of deflector wobble and the desirability of minimizing the deflector wobble angle is discussed in U.S. Pat. No., 4,239,326 issued Dec. 16, 1980 to Charles J. Kramer.
In the rigid mount shown in FIG. 1, a precision surface is machined on the top of an annular ring 10 projecting from a rigid mounting hub 12 which may be pressed fit to the shaft 14 of the motor 16. The hub has a length along the motor shaft that is about twice the shaft diameter and is made of the same stainless steel material as the motor shaft. The hologon disc unit wherein the grating facets are contained between glass plates 18, is tightly held against the precision machined hub surface of the ring 10 by a spring washer assembly 20 including a spring washer 22, a thrust ring 24, an O-ring 26 and a locking screw 28. A retainer 29 has a cylindrical post 30 from which a disc shaped flange 32 extends. A hub 33 at the center of the disc, which is part of the hologon disc unit 34, receives the motor shaft 14 and the post 30. The locking assembly fixes the rotational angular position of the disc unit 34, and the perpendicular angular orientation of the disc relative to the motor rotation axis 36 is determined by the accuracy of the rigid mounting hub 12, its precision surface of the ring 10 and the locking assembly 20. Such fixed, hard mounts as are shown in FIG. 1 have achieved deflector wobble angles from 20 to 40 arc seconds.
FIG. 2 illustrates a dynamic mount that employs a spherical bearing 40 which is keyed by a drive pin to the motor shaft 42. The hologon disc unit 44 (provided by the hologon grating in an air gap 46) between the glass cover discs (a substrate disc 48 and a cover disc 50) has a center which is fixedly connected to a raceway 52. The spherical curvature of the inner periphery of the raceway permits the disc to rotate freely about the center of the bearing 40 in any direction. The angular drive to the hologon unit 44 is obtained by the drive pin 54 which is disposed in a slot 56 in the raceway 52. The hologon unit can pivot freely in a direction about a plane perpendicular to the rotational axis of the shaft 42 in the direction indicated by the double-headed arrow 58 labeled "Gimbal Motion". The dynamic mount allows the deflector to approach a position with the disc unit 44 perpendicular to the rotational axis of the shaft 42. The centrifugal force which is developed as the disc rotates to bring it to the perpendicular position decreases as the disc unit 44 approaches perpendicular position so that the perpendicular position is approached asymptotically without ever actually reaching it. Typical performance for a dynamic mount such as shown in FIG. 2 is 20 to 40 arc seconds of fixed deflector wobble angle.
Both the dynamic mount of FIG. 2 and the fixed, rigid mount of FIG. 1, as well as variations thereof such as described in the above-referenced patents, have many problems associated therewith and do not achieve as low a disc wobble as desired, thus limiting hologon scanner apparatus to certain orientations in order to make them more insensitive to mechanical wobble. Such arrangements are shown, for example, in U.S. Pat. No. 4,243,293 issued Jan. 6, 1981 to Charles J. Kramer. It is a feature of this invention to provide greater flexibility and freedom of design of hologon laser scanner apparatus by reducing mechanical wobble.
The dynamic mounts provide best performance at high speeds of rotation (e.g. 12,000 rpm), since at low speeds the forces which provide the alignment of the disc to the rotational axis of the shaft are reduced. The fixed mount of FIG. 1 is fairly heavy, particularly because of the size of the hub which, while preventing movement makes the hub assembly heavy which limits the rotational speed of the hologon/motor assembly and makes it difficult to repair the ball bearings in the motor when they wear out.
In the dynamic mount, improper operation usually occurs as a result of the desire to minimize any radial play in the spherical bearing which would contribute to balancing problems. Radial play in the spherical bearing is minimized by having the bearing have a tight fit between the inner bearing member (the spherical bearing 40) and the outer bearing raceway 52. This tight fit increases the friction between the bearing surfaces and causes the bearing to stick at given positions. A lubricating film, such as Teflon has been used, but with such a film the bearing sometimes sticks as the hologon disc approaches the self-alignment position, since the alignment forces decrease asymptotically as the disc aligns itself.
Bearing sticking is exacerbated when the hologon unit must be run at both high and low rotation speeds because the fit between the bearing members is adjusted for balancing at the high rotation speed, which usually produces an internal bearing friction which is large relative to the lower alignment forces that are present at the lower rotation speeds. The alignment forces are proportional to the square of the rotation speed. Also, when the hologon unit is not turning, it can experience a shock load when the apparatus is struck or moved. This shock load can cause the hologon disc unit to misalign from its self-alignment position. If the hologon unit sits at this misaligned position for a long period of time, the bearing assembly can take a set which causes the hologon disc and its bearing assembly to stick in the misaligned orientation which is difficult to unstick.
It has been proposed to glue the hologon bearing assembly in order to lock it in position. Gluing is undesirable since the entire assembly becomes unusable because its orientation cannot be reset, as is required, since it changes overtime during operation of the apparatus. Moreover, locking of the hologon disc into its self-alignment position by putting glue into the bearing of the dynamic mount (FIG. 2) and letting it set up after the unit has rotated into its self-alignment position has been found to be impractical, since the glue tends to be spun out of the hologon disc by centrifugal force before the glue sets up and locks the bearing in the self-alignment position.
Accordingly, it is the principal object of the present invention to improve hologon scanner apparatus by providing an improved mounting for retaining a hologon disc unit on a drive shaft wherein the difficulties and disadvantages of prior fixed, hard mounts and dynamic mounts are eliminated while maintaining the benefits thereof.